Direct digital frequency synthesis is a technique for generating programmable frequency sinusoidal signals. In a direct digital frequency synthesizer (DDFS), an input digital signal is used to indicate, directly or indirectly, the desired output frequency. The DDFS provides an output signal that represents digital values for points along a sine wave. The rate at which these values cycle at the DDFS output determines the frequency of the signal.
For example such a DDFS can be advantageously used in a radio frequency (RF) application, such as an RF-to-IF mixer, as described in U.S. Pat. No. 6,778,117, invented by Richard A. Johnson. According to Nyquist's theorem, the DDFS must be clocked at a rate at least twice as high as the frequency of the signal represented. Since the DDFS provides a digital representation of the output clock signal, rather than the output clock signal itself, there is no mechanism for local oscillator signals to leak or radiate into other circuits, causing unwanted locking or spurs.
Recent advances in low voltage CMOS technology have allowed the generation of clock signals on-chip that are high enough to meet the Nyquist criterion for clocking a DDFS for many applications. What is needed are new ways of using a DDFS in these applications.